Some instant heat packs contain cellulose, iron, water, activated carbon, vermiculite and salt and produce heat from the exothermic oxidation of iron when exposed to air.

A second type generate heat through exothermic crystallisation of supersaturated solutions and are usually reusable. These can be recharged by boiling the warmers and allowing them to cool. Heating of these pads is triggered by snapping a small metal device buried in the pad which generates nucleation centers which initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.

Keep the heat pack insulated if possible. Use an insulated pouch, muff, covers, etc. This will make a big difference in the duration of the heat provided.

This second type typically has a shorter heat duration of 20 minutes to 2 hours.

A third type uses lighter fluid (lighter fuel) or LPG which is reacted with a platinum catalyst to release heat by oxidation reactions. These can be used on many occasions by simply refuelling.

Another type, known as HeatBands, consists of a thin band of a certain material that is worn around the neck. It works by insulating the blood vessels that are close to the skin’s surface at the wrist and by reflecting the body's own heat back to that area. It was originally developed for sufferers of Raynaud's disease.

A fourth type of handwarmer is battery operated. In this type, electrically resistive heating devices are used to convert electrical energy in the battery to thermal energy. Some use disposable batteries, but, others are rechargeable, like cellphones, and can be used for many hundreds of cycles with the same battery. This type is not disposable, and is more friendly to the environment.